Recently, as one type of an image forming apparatus, there has been proposed a multi-color image forming apparatus which is so arranged that, with a plurality of developing units respectively containing developers in different colors being disposed around a photosensitive drum, a series of image forming processings such as corona charging, exposure, partial erasing, developing, transfer and fixing, etc. is effected two times, thereby reproducing an image of one sheet of an original document in a plurality of colors.
In the known image forming apparatus as described above, however, particular paper passage means is required for guiding a copy paper sheet formed with the image at a first image forming process into a second image forming process, thus resulting in a large size of the apparatus and complication in its construction.
Moreover, in the above conventional arrangement, since the copy paper sheet continuously passes through the two image forming processes, and is subjected to mechanical and thermal stress during that time, it tends to be damaged through formation of wrinkles and curls, thereby giving rise to paper jamming or deviation in the position of copied images.
In order to solve these problems, the assignee of the present invention has proposed in U.S. patent application Ser. No. 148,423 filed on Jan. 25, 1988 a multi-color image forming apparatus having a copying mode (hereinbelow, refered to as a "simultaneous color copying mode") in which by selectively driving a plurality of developing units containing developers in different colors, respectively, during one cycle of copying operation, for example, a region from a forward edge of an original document to a first point is reproduced with a first color, a region from the first point to a second point is reproduced with a second color, and a region from the second point to a rear edge of the original document is reproduced with the first color again.
Meanwhile, in the developing units of the multi-color image forming apparatus having the simultaneous color copy mode, a magnet roller in which a plurality of magnets having axillary extending magnetic poles are arranged in a circumferential direction is disposed in a developing sleeve. The developing sleeve is rotationally driven at the time of development, but is stopped to rotate at the time of nondevelopment. Besides, positions of the magnetic poles of the magnet roller are changed over at the time of development and nondevelopment. Namely, at the time of development, a magnetic brush is brought into contact with a surface of an electrostatic latent image support member, while at the time of nondevelopment, the magnetic brush is substantially held out of contact with said surface such that supply of the developers to said surface from the developing units not in use is cut off, thereby preventing mixing of colors and thin colors in the image.
While, if changeover of the developing sleeve is performed concurrently with changeover of magnet roller upon the changeover from nondeveloping state to the developing state, a great centrifugal force due to not only a transport force based on rotation of the developing sleeve but a transport force based on displacement of the magnetic poles of the magnet roller is exerted on the developers temporarily.
Thus, the copying apparatus as mentioned above has such a drawback that some of the developers on the developing sleeve may scatter to the surface of the electrostatic latent image support member and scattering patterns made by the developers are formed in the vicinity of the boundary portions of the image so as to deteriorate the image quality.